The present invention relates to a thermistor material for a short range of low temperature use and a method of manufacturing the thermistor material, and more specifically relates to a thermistor material for a short rang of low temperature use preferable for temperature measurement in a temperature range from about −80° C. to about 500° C., and a method of manufacturing the thermistor material.
The thermistor refers to a resistor showing great electric resistance change against temperature change. Thermistors are classified into a NTC thermistor the electric resistance of which decreases with an increase in temperature, a PTC thermistor the electric resistance of which increases with increase in temperature, and a CRT thermistor the electric resistance of which drastically decreases at more than certain temperature. Among them, the NTC thermistor is used most frequently since its electric resistance value varies proportionally with temperature, and simple “thermistor” refers to the NTC thermistor.
A typically used thermistor includes an oxide composite containing two to four types of transition metal oxides such as oxides of Mn, Ni, Co, Fe, and Cu. A Pt lead is necessary to be bonded to a thermistor element having a predetermined shape in order to use the thermistor as any of various sensors (for example, a temperature sensor usable in a high temperature region). In a known method of bonding the Pt lead, the Pt lead and raw material powder are integrally molded and sintered. In another known method, an electrode is formed on a surface of a sintered body by printing, and the Pt lead is bonded to the electrode surface. The thermistor element having the Pt lead bonded thereto is typically used while being sealed by glass seal or a metal tube in order to suppress time-dependent variation of an electric resistance value due to a factor other than temperature change.
However, in the case where the Pt lead and the raw material powder are integrally molded and sintered, and if sintering temperature of the raw material powder is excessively high, the Pt lead is disadvantageously degraded during the sintering. Even if the thermistor element is sealed by glass seal or a metal tube, time-dependent variation of an electric resistance value disadvantageously occurs due to change in gas composition in a sealed space.
Various proposals have been made in order to overcome such disadvantages.
For example, Patent Literature 1 discloses a wide-range thermistor produced by adding 8.7 to 8.8 wt % silicon carbide, 29.1 to 29.3 wt % yttrium oxide, 0.8 wt % titanium boride, and 0.2 to 2.0 wt % metal boron to silicon nitride, mixing them together, and molding and sintering the resultant mixture.
Patent Literature 1 describes that the wide-range thermistor shows a linear relationship between temperature and logarithm of specific electric resistance in a range from room temperature (25° C.) to 1050° C.
Patent Literature 2 discloses a thermistor material for use in a reducing atmosphere such as hydrogen gas, which is produced by adding 30 wt % SiC powder and 6 wt % Y2O3 to Si3N4 powder and mixing them together, and molding and sintering the resultant mixture.
Patent Literature 2 describes that the thermistor material for use in a reducing atmosphere shows a deterioration rate of an electric resistance of 1% or less when the thermistor material is exposed for 1000 hr under a hydrogen atmosphere of 120° C.×10 atm.
Furthermore, Patent Literature 3 discloses a thermistor material containing silicon carbide and/or boron carbide as a conductive substance and an oxide matrix.
Patent Literature 3 describes that the thermistor material shows a change rate of an electric resistance value to an initial value of less than 1% after the lapse of 3000 hr at 500° C.
To achieve high measurement accuracy when the thermistor material is used for temperature measurement in a certain temperature range, it is necessary that
(a) the thermistor material has an appropriate electric resistance value in an operating temperature range, and
(b) the thermistor material has a linear relationship between temperature (T) or a reciprocal of temperature (1/T) and logarithm of an electric resistance value (log R) in an operating temperature range (i.e., the thermistor material has an appropriate temperature coefficient of resistance (B value)). In the present invention, a constant B approximated by R=Aexp(−BT) is defined as “temperature coefficient of resistance” or “thermistor constant.”
The thermistor material described in Patent Literature 1 has a temperature coefficient of resistance (B value) of less than 0.01, and therefore allows temperature measurement in a wide range from room temperature to about 1000° C. However, if this composite is directly used for temperature measurement in a low temperature region from about −80° C. to about 500° C., detection accuracy is disadvantageously low since electric resistance change has a small absolute value for the range of temperature.
In the case of the thermistor material described in Patent Literature 2, since the conductive material includes only SiC, the electric resistance value and the temperature coefficient of resistance are difficult to be adjusted together to values suitable for measurement in the short range of a low temperature region.
Similarly, in the case of the thermistor material described in Patent Literature 3, since the temperature coefficient of resistance is determined by properties of silicon carbide, the electric resistance value and the temperature coefficient of resistance are difficult to be adjusted together to values suitable for measurement in a low temperature region. In addition, since the thermistor material is hardly sinterable, high temperature sintering is required to produce a dense sintered body. This is because if the sintered body has a low density, the electric resistance value may become high, or electric resistance becomes unstable.